Salts of polyaspartic acid by high temperature reaction

ABSTRACT

Polyaspartate, useful for inhibition of incrustations due to materials causing hardness in water and of value in detergent formulations, can be prepared by reacting maleic acid or fumaric acid in a molar ratio of 1:1-2.1 at temperatures greater than 190° C., followed by conversion of the polymer formed in this reaction to a salt of polyaspartic acid by basic hydrolysis.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of pending U.S. applicationSer. No. 08/287,154 filed on Aug. 8, 1994, which is a continuation of08/199,652, filed on Feb. 22, 1994, now U.S. Pat. No. 5,367,047, whichwas a divisional of 08/007,376 filed on Jan. 21, 1993, now U.S. Pat. No.5,288,783; which was a continuation-in-part of 07/882,919 filed on May14, 1992, now abandoned.

FIELD OF THE INVENTION

This invention relates to a process for the production of polyasparticacid and its salts and the use of these materials.

DESCRIPTION OF RELATED ART

The salts of polyaspartic acid have been used for fertilizers, and scaleinhibition agents. They are particularly useful for the prevention ofscale deposition in boiler water, reverse osmosis membranes and indetergents. One of the primary characteristics that makes them valuablein this respect is the fact that they are readily biodegradable, whereasother materials that are presently used for this purpose are eitherslowly biodegradable, e.g. polyacrylic acid, or harmful to theenvironment, e.g. polyphosphoric acid.

Sodium polyaspartate was used in the prevention of boiler scale bychanging the crystal structure of calcium salts resulting in theformation of a soft scale (Sarig et al, The use of polymers forretardation of scale formation. Natl Counc Res Dev Rep! (Isr.), 150,1977). Polyaspartic acid, molecular weight (MW) 6,000, was found to besuperior to polyglutamate, MW 14,400, polyvinyl sulfonate, MW 5300, andpolyacrylic acid, MW 6,000, in that it gave 66% retardation of totalscale and 90% retardation of calcium sulfate scale. In addition, thescale formed in the presence of polyaspartate was softer than thatproduced in the presence of polyacrylate, polyglutamate and polyvinylsulfonate.

U.S. Pat. No. 4,839,461 discloses a method for making polyaspartic acidfrom maleic acid and ammonia by reacting these constituents in a 1:1-1.5molar ratio by raising the temperature to 120°-150° C. over a period of4-6 hours and maintaining it for 0-2 hours. It is further disclosed thattemperatures above 140°-160° C. result in elimination of CO₂, thusteaching degradation of the material. The molecular weight rangeobtained by this method was said to be 1,000-4,000 with a cluster at1,800-2,000. That patent states that this material is useful in theprevention of tarnishing glass and porcelain articles. Although notstated in this patent, it is known that this action would occur as aresult of the inhibition of calcium sulfate deposition. Harada, et al(Thermal polycondensation of free amino acids with polyphosphoric acid.Origins Prebiol. systems Their Mol Matrices, Proc. Conf., WakullaSprings, Fla., 289, 1963) obtained polyaspartic acid from aspartic acidand phosphoric acid at temperatures over 100° C. over a time period of50-250 hrs, but required temperatures over 170° without phosphoric acidbeing present.

U.S. Pat. No. 5,057,597 discloses a method for the polycondensation ofaspartic acid to produce polyaspartic acid by heating the aspartic acidin a fluidized bed reactor to 221° C. for a period of 3-6 hours in anitrogen atmosphere followed by conventional alkaline hydrolysis.

Kovacs et al. (J. Org. Chem., 25 1084 1961!) prepared polyaspartic acidby heating aspartic acid to 200° C. in vacuo for a period of 120 hoursor in boiling tetralin over a period of 100 hours. Kovacs et al, showedthat the intermediate formed in the thermal polymerization of asparticacid was polysuccinimide.

Frankel et al. (J. Org. Chem., 16, 1513 1951!) prepared polyasparticacid by heating the benzyl ester of N-carboxyanhydroaspartic acidfollowed by saponification.

Dessaigne (Comp. rend. 31, 432-434 1850!) prepared condensation productswhich gave aspartic acid on treatment with nitric or hydrochloric acidby dry distillation of the acid ammonium salts of malic fumaric ormaleic acid at unspecified times and temperatures.

SUMMARY OF THE INVENTION

Polymers of aspartic acid which are suitable for the prevention of scalemay be obtained by reacting maleic acid and ammonia in a molar ratio of1:1-2.1 at 200°-300° C. and then converting the polysuccinimide formedto a salt of polyaspartic acid by hydrolysis with a metal hydroxide. Thereaction is carried out by the addition of water to maleic anhydride,thus forming maleic acid, or to maleic acid followed by addition of theappropriate amount of ammonia in the form of gaseous ammonia or as itsaqueous solution. This solution is then heated to remove water. A meltof the maleic acid and ammonia is formed and water removal continues asthe reaction proceeds and the temperature is brought to 200°-300° C.When the theoretical quantity of water formed in the production ofpolysuccinimide has been removed, which may occur in less than 5minutes, the reaction mixture is allowed to cool. The polysuccinimideformed can be used to make other useful products or can be hydrolyzedwith metal hydroxides to provide the appropriate salt of polyasparticacid. Solutions of the salts of polyaspartic acid formed in this mannerhave the same scale inhibition performance and molecular weight range asdo the polymers formed by the thermal polymerization of aspartic aciditself. Further manipulation to remove the water or the salts can becarried out to provide water free powders of the salts or the free acid.

The polyaspartic acid provided by the present invention is suitable forinhibition of scale deposition, whereas the methods previously used toproduce polyaspartic acid did not provide a polymer of sufficientmolecular weight to prevent scale formation.

The object of this invention is to provide a means of preparingpolysuccinimide. A further object of this invention is to provide ameans of preparing salts of polyaspartic acid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the effect of additives on the inhibition of calciumcarbonate precipitation.

FIG. 2 shows the effect of thermally polymerized mono-ammonium maleatesalts as calcium scale inhibitors.

FIG. 3 shows the effect of thermally polymerized mono-ammonium fumaratesalts as calcium scale inhibitors.

FIG. 4 shows the effect of thermally polymerized di-ammonium fumarateand maleate salts as calcium scale inhibitors.

FIG. 5 shows the calibration of a molecular weight column.

FIG. 6 shows the molecular weight determination of polymers formed inExamples 2, 4, 10, and 11.

FIG. 7 shows the molecular weight determination of polymers formed inExamples 6, 8, 12, and 13.

FIG. 8 shows the molecular weight determination of polymers formed inExamples 3 and 7

DETAILED DESCRIPTION OF THE EMBODIMENTS

As opposed to the teachings of U.S. Pat. No. 4,839,461, I have foundthat, although the use of the polyaspartic made by the process is saidto be useful in the prevention of hardness deposits, no actualexperimentation to confirm this finding is reported. In fact, uponcareful repetition of the procedures of U.S. Pat. No. 4,839,461, theresults below clearly demonstrate that polymers of aspartic acidprepared by heating the ammonium salts of maleic acid at 140°-150° C.for 4 to 6 hours did not give a polymer that was active as a calciumscale inhibitor. Further, when calculations of the theoretical weight ofpolysuccinimide (molecular weight 97) formed in Example 1 indicates thatthe reaction was not taken to completion under the conditions described.

EXAMPLE 1 Thermal Polymerization of L-Aspartic Acid at 240°-250° C.

Aspartic acid, 133 g, was tumbled under nitrogen at 100 Torr, at240°-250° C. for 1.5 hours to give a pink powder weighing 97.3 g. Thissolid was slurried in 200 ml of water at 25° C. and a solution of 40 gof water containing 40.0 g of sodium hydroxide was added over a periodof 15 minutes with intermittant cooling to keep the temperature between60° and 70° C. The resultant clear red-brown solution, pH 12.0, wasadjusted to pH 7.0 by the addition of 1.5 g of citric acid and contained25% solids.

The sodium polyaspartate was tested for inhibition of calcium carbonateprecipitation by the calcium drift assay. In this assay a supersaturatedsolution of calcium carbonate is formed by adding 29.1 ml of 0.55M NaCland 0.01M KCl to 0.15 ml of 1.0M CaCl₂ and 0.3 ml of 0.5M NaHCO₃. Thereaction is initiated by adjusting the pH to 7.5-8.0 by titration with1N NaOH and addition of the material to be tested for inhibition ofCaCO₃ precipitation at a level of 1.7 ppm. At three minutes, 10 mg ofCaCO₃ is added and the pH is recorded. The decrease in pH is directlycorrelated to the amount of CaCO₃ that precipitates. The effectivenessof the inhibition is compared to that of sodium polyacrylate, usedcommercially for the purpose of preventing scale formation.

FIG. 1 shows the effect of no additive in this test compared withpolyacrylate, chemically synthesized L-α-polyaspartate and thepolyaspartate prepared in this Example. Both thermally prepared andchemically synthesized polyaspartate were very close to polyacrylate bythe calcium drift assay when all materials were tested at 1.7 ppm ofadditive.

EXAMPLE 2 Thermal Polymerization of Mono-Ammonium Maleate at 145°-150°C.

Following the examples of U.S. Pat. No. 4,839,461, a slurry of 9.8 g(0.1 mole) maleic anhydride was dissolved in 20 ml water at 80°-95° C.and stirred for 30 minutes while allowing the mixture to cool to 25° C.To this colorless solution at 25° C. was added 13 g of 30% aqueoussolution of ammonium hydroxide (0.11 mol NH₃) to give a colorlesssolution. This solution was boiled to dryness over a period of 30minutes at approximately 100°-115° C. to give a white crystalline solid.The solid was tumbled under nitrogen at 100 Torr, at 145°-150° C. for 4hours to give a water insoluble, pinkish-tan brittle glasslike solidweighing 11.4 g. This solid was dissolved in 26.2 g of an aqueoussolution containing 1.36 g of sodium hydroxide to form a clear red-brownsolution, pH 7.0, containing 25% solids.

FIG. 2 shows a plot of the data obtained in this example compared tothat of the no additive assay and the thermally prepared polyaspartate.The material obtained at 145°-150° C. is no better than no additve whentested at 1.7 ppm.

EXAMPLE 3 Thermal Polymerization of Mono-Ammonium Maleate at 190°-200°C.

A slurry of 9.8 g (0.1 mole) maleic anhydride was dissolved in 20 mlwater at 80°-95° C. and stirred for 30 minutes while allowing themixture to cool to 25° C. To this colorless solution at 25° C. was added13 g of 30% aqueous solution of ammonium hydroxide (0.11 mol NH₃) togive a colorless solution. This solution was boiled to dryness over aperiod of 30 minutes at approximately 100°-115°-C. to give a whitecrystalline solid. The solid was tumbled under nitrogen at 100 Torr, at190°-200° C. for 4 hours to give a water insoluble, pinkish-tan brittleglasslike solid weighing 10.6 g. This solid was dissolved in 35.4 g ofan aqueous solution containing 1.9 g of sodium hydroxide to form a clearred-brown solution, pH 9.0, containing 25% solids.

FIG. 2 shows that polyaspartic acid of this example in the calcium driftassay of Example 1 at 1.7 ppm much improved compared to the material ofExample 2.

EXAMPLE 4 Thermal Polymerization of Mono-Ammonium Maleate at 240°-250°C.

A slurry of 9.8 g (0.1 mole) maleic anhydride was dissolved in 20 mlwater at 80°-95° C. and stirred for 30 minutes while allowing themixture to cool to 25° C. To this colorless solution at 25° C. was added13 g of 30% aqueous solution of ammonium hydroxide (0.11 mol NH₃) togive a colorless solution. This solution was boiled to dryness over aperiod of 30 minutes at approximately 100°-115° C. to give a whitecrystalline solid. The solid was tumbled under nitrogen at 100 Torr, at240°-250° C. for 1.5 hours to give a water insoluble, pinkish-tanbrittle glasslike solid weighing 9.6 g. This solid was dissolved in 36.0g of an aqueous solution containing 4.0 g of sodium hydroxide to form aclear red-brown solution, pH 12.0. To this solution was added 0.25 gcitric acid to adjust the pH to 8.5 and the resultant solution contained25% solids.

FIG. 2 shows that the polyaspartic acid of this example in the calciumdrift assay of Example 1 at 1.7 ppm was equivalent to that of thermallyprepared polyaspartate.

EXAMPLE 5 Thermal Polymerization of Mono-Ammonium Maleate at 300° C.

A slurry of 9.8 g (0.1 mole) maleic anhydride was dissolved in 20 mlwater at 80°-95° C. and stirred for 30 minutes while allowing themixture to cool to 25° C. To this colorless solution at 25° C. was added13 g of 30% aqueous solution of ammonium hydroxide (0.11 mol NH₃) togive a colorless solution. This solution was boiled to dryness over aperiod of 30 minutes at approximately 100°-115° C. to give a whitecrystalline solid. The solid was tumbled at 300° C. for 5 minutes togive a water insoluble, brick-red brittle glasslike solid weighing 9.6g. This solid was dissolved in 40.0 g of an aqueous solution containing3.8 g of sodium hydroxide to form a clear red-brown solution, pH 9.0,containing 25% solids.

FIG. 2 shows that the polyaspartic acid of this example in the calciumdrift assay of Example 1 at 1.7 ppm was equivalent to that of thermallyprepared polyaspartate.

In summary, polyaspartic acid prepared at 145°-150° C. from maleicanhydride and ammonia was ineffective as a scale inhibitor while thatprepared at 190°-200° C. was nearly as effective as thermalpolyaspartate and that prepared at 240° or 300° C. was equivalent tothermal polyaspartic as a scale inhibitor. The time required forpolymerization was reduced from 4-8 hours to between 5 minutes and 1.5hours, thus providing a significant improvement in the economy ofindustrial production.

EXAMPLE 6 Thermal Polymerization of Mono-Ammonium Fumarate at 145°-150°C.

Following the examples of U.S. Pat. No. 4,839,461, a slurry of 11.6 g(0.1 mole) fumaric acid was dissolved in 30 ml water was mixed with 13 gof 30% aqueous solution of ammonium hydroxide (0.11 mol NH₃). Carefullywarming the slurry to boiling gave a clear solution. This solution wasboiled to dryness over a period of 15 minutes to give a whitecrystalline solid. The solid was tumbled under nitrogen at 100 Torr, at145°-150° C. for 8 hours to give an off-white glasslike solid weighing13.2 g. This solid was dissolved in 40 g of an aqueous solutioncontaining 4.0 g of sodium hydroxide to form a pale yellow solution, pH8.5, containing 25% solids.

FIG. 3 shows a plot of the data obtained in the calcium drift assay onthe material obtained in this example. The material was only slightlybetter than no additve when tested at 1.7 ppm.

EXAMPLE 7 Thermal Polymerization of Mono-Ammonium Fumarate at 190°-200°C.

A slurry of 11.6 g (0.1 mole) fumaric acid was dissolved in 30 ml waterwas mixed with 13 g of 30% aqueous solution of ammonium hydroxide (0.11mol NH₃). Carefully warming the slurry to boiling gave a clear solution.This solution was boiled to dryness over a period of 15 minutes to givea white crystalline solid. The solid was tumbled under nitrogen at 100Torr, at 190°-200° C. for 4 hours to give a water insoluble, tanglasslike solid weighing 12.0 g. This solid was dissolved in 40 g of anaqueous solution containing 4.0 g of sodium hydroxide to form a paleyellow solution, pH 7.0, containing 25% solids.

FIG. 3 shows a plot of the data obtained in the calcium drift assay onthe material obtained in this example. The material was only slightlybetter than no additive when tested at 1.7 ppm.

EXAMPLE 8 Thermal Polymerization of Mono-Ammonium Fumarate at 240°-250°C.

A slurry of 11.6 g (0.1 mole) fumaric acid was dissolved in 30 ml waterwas mixed with 13 g of 30% aqueous solution of ammonium hydroxide (0.11mol NH₃). Carefully warming the slurry to boiling gave a clear solution.This solution was boiled to dryness over a period of 15 minutes to givea white crystalline solid. The solid was tumbled under nitrogen at 100Torr, at 240°-250° C. for 1.5 hours to give a water insoluble, darkbrown glasslike solid weighing 9.3 g. This solid was dissolved in 40 gof an aqueous solution containing 4.0 g of sodium hydroxide to form aclear brown solution, pH 8.0, containing 25% solids.

FIG. 3 shows a plot of the data obtained in the calcium drift assay onthe material obtained in this example. The material was much better thanthat prepared in Example 6 when tested at 1.7 ppm.

EXAMPLE 9 Thermal Polymerization of Mono-Ammonium Fumarate at 300° C.

A slurry of 11.6 g (0.1 mole) fumaric acid was dissolved in 30 ml waterwas mixed with 13 g of 30% aqueous solution of ammonium hydroxide (0.11mol NH₃). Carefully warming the slurry to boiling gave a clear solution.This solution was boiled to dryness over a period of 15 minutes to givea white crystalline solid. The solid was tumbled at 300° C. for 5minutes to give a water insoluble, dark brown glasslike solid weighing9.8 g. This solid was dissolved in 40 g of an aqueous solutioncontaining 3.8 g of sodium hydroxide to form a clear brown solution, pH9.0, containing 25% solids.

FIG. 3 shows a plot of the data obtained in the calcium drift assay onthe material obtained in this example. The material was much better thanthat prepared in Example 6 when tested at 1.7 ppm.

In summary, thermally polymerized mono-ammonium fumarate providedpolyaspartate prepared at 145°-150° C. and at 190°-200° C. which wasonly slightly active in scale inhibition while that prepared at 240° C.and at 300° C. were active but less active than thermal polyaspartate asscale inhibitors.

EXAMPLE 10 Thermal Polymerization of Di-Ammonium Maleate at 135°-140° C.

Following the examples of U.S. Pat. No. 4,839,461, a solution of 1.96 g(0.02 mole) maleic anhydride was dissolved in 1 ml water at 50°-60° C.and stirred for 30 minutes while allowing the mixture to cool to 25° C.To this colorless solution at 2° C. was added 2.4 g of 30% aqueoussolution of ammonium hydroxide (0.022 mol NH₃) to give a colorlesssolution. This solution was boiled to dryness over a period of 30minutes at approximately 100°-120° C. and 10-20 Torr, to give a whitecrystalline solid. The solid was tumbled under nitrogen at 100 Torr, at135°-140° C. for 8 hours to give a water insoluble, pinkish-tan brittleglasslike solid weighing 2.7 g. This solid was dissolved in 6.6 g of anaqueous solution containing 0.8 g of sodium hydroxide to form a clearorange solution, pH 7.0, containing 25% solids.

FIG. 4 shows a plot of the data obtained in this example compared tothat of the no additive assay and the thermally prepared polyaspartate.The material obtained at 135°-145° C. is not as good as no additive whentested at 1.7 ppm.

EXAMPLE 11 Thermal Polymerization of Di-Ammonium Maleate at 240°-250° C.

A solution of 9.8 g (0.1 mole) maleic anhydride was dissolved in 20 mlwater at 50°-60° C. and stirred for 30 minutes while allowing themixture to cool to 25° C. To this colorless solution at 25° C. was added26 g of 30% aqueous solution of ammonium hydroxide (0.22 mol NH₃) togive a colorless solution. This solution was boiled to dryness over aperiod of 30 minutes at approximately 100°-120° C. and 10-20 Torr, togive a white crystalline solid. The solid was tumbled under nitrogen at100 Torr, at 240°-250° C. for 1.5 hours to give a water insoluble,red-brown brittle glasslike solid weighing 9.4 g. This solid wasdissolved in 40 g of an aqueous solution containing 3.8 g of sodiumhydroxide to form a clear red-brown solution, pH 7.0, containing 25%solids.

FIG. 4 shows a plot of the data obtained in this example compared tothat of the no additive assay and the thermally prepared polyaspartate.The material of this example is equivalent to that of thermalpolyaspartate when tested at 1.7 ppm.

EXAMPLE 12 Thermal Polymerization of Di-Ammonium Fumarate at 140°-150°C.

A slurry of 11.6 g (0.1 mole) fumaric acid was dissolved in 30 ml waterwas mixed with 26 g of 30% aqueous solution of ammonium hydroxide (0.22mol NH₃). Carefully warming the slurry to boiling gave a clear solution.This solution was boiled to dryness over a period of 15 minutes to givea white crystalline solid. The solid was tumbled under nitrogen at 100Torr, at 140°-150° C. for 8 hours to give a water insoluble, brown,glasslike solid weighing 14 g. This solid was dissolved in 100 g of anaqueous solution containing 2.0 g of sodium hydroxide to form a paleyellow solution, pH 7.0, containing 25% solids.

FIG. 4 shows a plot of the data obtained in the calcium drift assay onthe material obtained in this example. The material was only slightlybetter than no additve when tested at 1.7 ppm.

EXAMPLE 13 Thermal Polymerization of Di-Ammonium Fumarate at 235°-245°C.

A slurry of 11.6 g (0.1 mole) fumaric acid was dissolved in 30 ml waterwas mixed with 26 g of 30% aqueous solution of ammonium hydroxide (0.22mol NH₃). Carefully warming the slurry to boiling gave a clear solution.This solution was boiled to dryness over a period of 15 minutes to givea white crystalline solid. The solid was tumbled under nitrogen at 100Torr, at 235°-245° C. for 1.5 hours to give a water insoluble, brown,glasslike solid weighing 9.0 g. This solid was dissolved in 100 g of anaqueous solution containing 2.0 g of sodium hydroxide to form a paleyellow solution, pH 8.5, containing 25% solids.

FIG. 4 shows a plot of the data obtained in the calcium drift assay onthe material obtained in this example. The material was only slightlybetter than no additve when tested at 1.7 ppm.

EXAMPLE 14 Molecular Weight Analysis of Polyaspartate Prepared inVarious Ways

Molecular weight determination of the materials prepared in theforegoing examples and commercially available materials was made bychromatography on a 1 cm×18 cm, Sephadex G-50 column in a mobile phaseof 0.02M sodium phosphate buffer, pH 7.0, running at 0.5 ml/min, withdetection in the UV at 240 nm. The sample size ranged from 0.01 to 0.5mg/ml.

FIG. 5 shows the results of sodium polyaspartate, 13,000 m.w., fromSigma, I; sodium polyaspartate, 7,500 m.w., from Sigma, II; and sodiumpolyaspartate, Example 1, II. m.w. 5,000, from Sigma.

FIG. 6 shows the result of Example 4 as "a"; Example 11 as "b"; Example2 as "c" and Example 10 as "d". With maleic acid and ammonia reactions,temperatures of 240° C. gave molecular weights over a broad rangecentered at 7,000-8,000 while temperatures of 135°-150° C. gavemolecular weights over a broad range centered at 2,000.

FIG. 7 shows the result of Example 8 as "e"; Example 13 as "f"; Example6 as "g" and Example 12 as "h". With fumaric acid and ammonia reactions,temperatures of 240° C. gave molecular weights over a broad rangecentered at 7,000-8,000 while temperatures of 140°-150° C. gavemolecular weights over a broad range centered at 2,000.

FIG. 8 shows the result of Example 3 as "i"; Example 7 as "j".Temperatures of 190°-200° C. gave molecular weights for maleate over abroad range centered at 7,000-8,000 while temperatures of 190°-200° C.for fumarate gave molecular weights over a broad range centered at2,000.

I claim:
 1. A process for the preparation of a salt of polyaspartic acid comprising reacting maleic acid and ammonia in a molar ratio of 1:1-2.1, heating to remove water forming a melt of maleic acid and ammonia, removing water as the reaction proceeds, bringing the temperature to 190° to 300° C. and converting the resultant polymer into a salt of polyaspartic acid by basic hydrolysis.
 2. A process according to claim 1, wherein the temperature is brought to 200° to 300° C.
 3. A process according to claim 1, wherein the temperature is brought to 240° to 300° C.
 4. A process according to claim 1, wherein the temperature is brought to 190° to 300° C. for a time of four hours or less.
 5. A process according to claim 1, wherein the basic hydrolysis is performed by adding a metal hydroxide to the resultant polymer.
 6. A process for the preparation of a salt of polyaspartic acid comprising reacting fumaric acid and ammonia in a molar ratio of 1:1-2.1, heating the obtained solution to remove water, removing water as the reaction proceeds, bringing the temperature to 190° to 300° C. and converting the resultant polymer into a salt of polyaspartic acid by basic hydrolysis.
 7. A process according to claim 6, wherein the temperature is brought to 200° to 300° C.
 8. A process according to claim 6, wherein the temperature is brought to 240° to 300° C.
 9. A process according to claim 6, wherein the temperature is brought to 190° to 300° C. for a time of four hours or less.
 10. A process according to claim 6, wherein the basic hydrolysis is performed by adding a metal hydroxide to the resultant polymer. 